The present invention relates generally to improvements in light responsive switching devices and it relates more particularly to an improved switching device responsive to the ambient light for controlling the energization of street lighting lamps or other loads.
It is a common practice in the control of the lighting and extinguishing of street lamps to connect each of the lamps to a source of current through an individual light responsive switch so that the respective lamp is energized and de-energized with the ambient light falling below and exceeding pre-determined values respectively. A photoresponsive switch device which has heretofore been widely used for the above purposes is of the type including a snap type thermostatic switch through which the load is connected to a source of current, the switch temperature responsive element being exposed to and heated by a heater resistor which is connected through an ambient light exposed photoconductor to the current source so that the heater is controlled by the ambient light and, in turn, controls the opening and closing of the switch with increased and decreased ambient light values. While this type of switch, as presently available, has many desirable features it possesses numerous drawbacks and disadvantages.
The light responsive switch devices presently employed have an inherent differential between the make and break of the contacts. This differential, when the relay is installed in a conventional street lighting circuit is translated into a turn-off/turn-on ratio of 2:1 to 5:1 for most photocontrols. If it is desired to turn on a street light a one foot candle ambient light at night, the conventional photocontrol will turn off at from 2 to 5 foot candles in the morning. This is highly uneconomical and inefficient and represents a useful lighting time and a great waste of power. With the constant increase in the cost of energy and the decrease in the availability thereof, the above waste becomes more intolerable.
Furthermore, a photocell ages from the internally generated heat and as it ages the cell resistance usually increases. As a result the switch device employing such a cell will turn on earlier each night and turn off later each morning. This also represents a significant waste of power and gets progressively worse until the photocontrol eventually fails in the "on" position.